Pregnancy test
Encyclopedia
A pregnancy test attempts to determine whether or not a woman is pregnant
.These markers are found in urine and blood, and pregnancy tests require sampling one of these substances. The first of these markers to be discovered, human chorionic gonadotropin (hCG), was discovered in 1930 to be produced by the trophoblast cells of the fertilised ovum (blastocyst). While hCG is a reliable marker of pregnancy, it cannot be detected until after implantation: this results in false negatives if the test is performed during the very early stages of pregnancy. Obstetric ultrasonography
may also be used to detect pregnancy. Obstetric ultrasonography was first practiced in the 1960s; the first home test kit for hCG was released in the mid-1970s.
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Records of attempts at pregnancy testing have been found as far back as the ancient Greek
and ancient Egypt
ian cultures. The ancient Egyptians watered bags of wheat and barley with the urine of a possibly pregnant woman. Germination indicated pregnancy. The type of grain that sprouted was taken as an indicator of the fetus's sex. Hippocrates
suggested that a woman who had missed her period should drink a solution of honey in water at bedtime: resulting abdominal distention and cramps would indicate the presence of a pregnancy. Avicenna
and many physicians after him in the Middle Ages
performed uroscopy
, a nonscientific method to evaluate urine.
Selmar Aschheim
and Bernhard Zondek
introduced testing based on the presence of human chorionic gonadotropin (hCG) in 1928. Early studies of hCG had concluded that it was produced by the pituitary gland. In the 1930s, Georgeanna Joness discovered that hCG was produced not by the pituitary gland, but by the placenta. This discovery was important in relying on hCG as an early marker of pregnancy. In the Aschheim and Zondek test, an infantile female mouse
was injected subcutaneously with urine of the person to be tested, and the mouse later was killed and dissected. Presence of ovulation
indicated that the urine contained hCG and meant that the person was pregnant. A similar test was developed using immature rabbits
. Here, too, killing the animal to check her ovaries was necessary. An improvement arrived with the frog test, introduced by Lancelot Hogben
, which still was used in the 1950s and allowed the frog to remain alive and be used repeatedly: a female frog
was injected with serum or urine of the patient; if the frog produced eggs
within the next 24 hours, the test was positive. This was called the Bufo test, named after the toad genus Bufo
, which was originally used for the test. Other species of toads and frogs have been used later on.
Direct measurement of antigen
s, such as hCG, was made possible with the invention of the radioimmunoassay
in 1959. Radioimmunoassays require sophisticated apparatus and special radiation precautions and are expensive. In the 1970s, the discovery of monoclonal antibodies
led to the development of the relatively simple and cheap immunoassay
s, such as agglutination-inhibition-based assays and sandwich ELISA, used in modern home pregnancy tests.
for early pregnancy factor
(EPF). EPF can be detected in blood within 48 hours of fertilization
. However, testing for EPF is expensive and time-consuming.
Most chemical tests for pregnancy look for the presence of the beta subunit of hCG or human chorionic gonadotropin in the blood or urine. hCG can be detected in urine or blood after implantation, which occurs six to twelve days after fertilization. Quantitative blood (serum beta) tests can detect hCG levels as low as 1 mIU/mL, while urine test strip
s have published detection thresholds of 20 mIU/mL to 100 mIU/mL, depending on the brand. Qualitative blood tests generally have a threshold of 25 mIU/mL, and so are less sensitive than some available home pregnancy tests. Most home pregnancy tests are based on lateral-flow
technology.
With obstetric ultrasonography
the gestational sac
sometimes can be visualized as early as four and a half weeks of gestation
(approximately two and a half weeks after ovulation) and the yolk sac
at about five weeks' gestation. The embryo can be observed and measured by about five and a half weeks. The heartbeat may be seen as early as six weeks, and is usually visible by seven weeks' gestation.
A systematic review published in 1998 showed that home pregnancy test kits, when used by experienced technicians, are almost as accurate as professional laboratory testing (97.4%). When used by consumers, however, the accuracy fell to 75%: the review authors noted that many users misunderstood or failed to follow the instructions included in the kits. Improper usage may cause both false negatives and false positives.
readings can occur when testing is done too early. Quantitative blood tests and the most sensitive urine tests usually detect hCG shortly after implantation, which can occur anywhere from 6 to 12 days after ovulation
. Less sensitive urine tests and qualitative blood tests may not detect pregnancy until three or four days after implantation. Menstruation
occurs on average 14 days after ovulation, so the likelihood of a false negative is low once a menstrual period is late.
Ovulation may not occur at a predictable time in the menstrual cycle
, however. A number of factors may cause an unexpectedly early or late ovulation, even for women with a history of regular menstrual cycles. Using ovulation predictor kits (OPKs), or charting the fertility signs
of cervical
mucus or basal body temperature
give a more accurate idea of when to test than day-counting alone.
The accuracy of a pregnancy test is most closely related to the day of ovulation, not of the act of intercourse or insemination that caused the pregnancy. It is normal for sperm to live up to five days in the fallopian tubes, waiting for ovulation to occur. It could take up to twelve further days for implantation to occur, meaning even the most sensitive pregnancy tests may give false negatives up to seventeen days after the act that caused the pregnancy. Because some home pregnancy tests have high hCG detection thresholds (up to 100 mIU/mL), it may take an additional three or four days for hCG to rise to levels detectable by these tests — meaning false negatives may occur up to three weeks after the act of intercourse or insemination that causes pregnancy.
test results may occur for several reasons. These include: errors of test application, use of drugs containing the assay molecule, and non-pregnant production of the assay molecule.
Spurious evaporation lines may appear on many home pregnancy tests if read after the suggested 3–5 minute window or reaction time, independent of an actual pregnancy. False positives may also appear on tests used past their expiration date.
A woman who has been given an hCG injection as part of infertility
treatment will test positive on pregnancy tests that assay hCG, regardless of her actual pregnancy status. However, some infertility drugs (e.g., clomid) do not contain the hCG hormone.
Some diseases of the liver
, cancer
s, and other medical conditions may produce elevated Human chorionic gonadotropin|hCG and thus cause a false positive pregnancy test. These include choriocarcinoma
and other germ cell tumor
s, IgA deficiencies
, heterophile antibodies
, enterocystoplasties, gestational trophoblastic disease
s (GTD), and gestational trophoblastic neoplasms (GTN).
or a possible ectopic pregnancy
.
Ultrasound
is also a common tool for determining viability. A lower than expected heart rate or missed development milestones may indicate a problem with the pregnancy. Diagnosis should not be made from a single ultrasound, however. Inaccurate estimations of fetal age and inaccuracies inherent in ultrasonic examination may cause a scan to be interpreted negatively. If results from the first ultrasound scan indicate a problem, repeating the scan 7–10 days later is reasonable practice.
Pregnancy
Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...
.These markers are found in urine and blood, and pregnancy tests require sampling one of these substances. The first of these markers to be discovered, human chorionic gonadotropin (hCG), was discovered in 1930 to be produced by the trophoblast cells of the fertilised ovum (blastocyst). While hCG is a reliable marker of pregnancy, it cannot be detected until after implantation: this results in false negatives if the test is performed during the very early stages of pregnancy. Obstetric ultrasonography
Obstetric ultrasonography
Obstetric sonography is the application of medical ultrasonography to obstetrics, in which sonography is used to visualize the embryo or foetus in its mother's uterus...
may also be used to detect pregnancy. Obstetric ultrasonography was first practiced in the 1960s; the first home test kit for hCG was released in the mid-1970s.
History
Ancient Greece
Ancient Greece is a civilization belonging to a period of Greek history that lasted from the Archaic period of the 8th to 6th centuries BC to the end of antiquity. Immediately following this period was the beginning of the Early Middle Ages and the Byzantine era. Included in Ancient Greece is the...
and ancient Egypt
Ancient Egypt
Ancient Egypt was an ancient civilization of Northeastern Africa, concentrated along the lower reaches of the Nile River in what is now the modern country of Egypt. Egyptian civilization coalesced around 3150 BC with the political unification of Upper and Lower Egypt under the first pharaoh...
ian cultures. The ancient Egyptians watered bags of wheat and barley with the urine of a possibly pregnant woman. Germination indicated pregnancy. The type of grain that sprouted was taken as an indicator of the fetus's sex. Hippocrates
Hippocrates
Hippocrates of Cos or Hippokrates of Kos was an ancient Greek physician of the Age of Pericles , and is considered one of the most outstanding figures in the history of medicine...
suggested that a woman who had missed her period should drink a solution of honey in water at bedtime: resulting abdominal distention and cramps would indicate the presence of a pregnancy. Avicenna
Avicenna
Abū ʿAlī al-Ḥusayn ibn ʿAbd Allāh ibn Sīnā , commonly known as Ibn Sīnā or by his Latinized name Avicenna, was a Persian polymath, who wrote almost 450 treatises on a wide range of subjects, of which around 240 have survived...
and many physicians after him in the Middle Ages
Middle Ages
The Middle Ages is a periodization of European history from the 5th century to the 15th century. The Middle Ages follows the fall of the Western Roman Empire in 476 and precedes the Early Modern Era. It is the middle period of a three-period division of Western history: Classic, Medieval and Modern...
performed uroscopy
Uroscopy
Uroscopy is the historic medical practice of visually examining a patient's urine for pus, blood, or other symptoms of disease. It dates back to ancient Egypt, Babylon, and India. It was particularly emphasized in Byzantine medicine....
, a nonscientific method to evaluate urine.
Selmar Aschheim
Selmar Aschheim
Selmar Aschheim was a German gynecologist who was a native of Berlin. In 1902 he received a doctorate of medicine in Freiburg, and later became director of the laboratory of the Universitäts-Frauenklinik at the Berlin Charité. In 1930 Aschheim attained the chair of biological research in...
and Bernhard Zondek
Bernhard Zondek
Bernhard Zondek was a German-Jewish gynecologist who developed the first reliable pregnancy test in 1928.-Biography:...
introduced testing based on the presence of human chorionic gonadotropin (hCG) in 1928. Early studies of hCG had concluded that it was produced by the pituitary gland. In the 1930s, Georgeanna Joness discovered that hCG was produced not by the pituitary gland, but by the placenta. This discovery was important in relying on hCG as an early marker of pregnancy. In the Aschheim and Zondek test, an infantile female mouse
Mouse
A mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles...
was injected subcutaneously with urine of the person to be tested, and the mouse later was killed and dissected. Presence of ovulation
Ovulation
Ovulation is the process in a female's menstrual cycle by which a mature ovarian follicle ruptures and discharges an ovum . Ovulation also occurs in the estrous cycle of other female mammals, which differs in many fundamental ways from the menstrual cycle...
indicated that the urine contained hCG and meant that the person was pregnant. A similar test was developed using immature rabbits
Rabbit test
The rabbit test was an early pregnancy test developed in 1927 by Bernhard Zondek and Selmar Aschheim. The original test actually used mice. The test consisted of injecting the tested woman's urine into a female rabbit, then examining the rabbit's ovaries a few days later, which would change in...
. Here, too, killing the animal to check her ovaries was necessary. An improvement arrived with the frog test, introduced by Lancelot Hogben
Lancelot Hogben
Lancelot Thomas Hogben FRS was a versatile British experimental zoologist and medical statistician. He is best known for developing Xenopus laevis as a model organism for biological research in his early career, attacking the eugenics movement in the middle of his career, and popularising books on...
, which still was used in the 1950s and allowed the frog to remain alive and be used repeatedly: a female frog
Frog
Frogs are amphibians in the order Anura , formerly referred to as Salientia . Most frogs are characterized by a short body, webbed digits , protruding eyes and the absence of a tail...
was injected with serum or urine of the patient; if the frog produced eggs
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
within the next 24 hours, the test was positive. This was called the Bufo test, named after the toad genus Bufo
Bufo
Bufo is a large genus of about 150 species of true toads in the amphibian family Bufonidae. Bufo is a Latin word for toad.- Description :...
, which was originally used for the test. Other species of toads and frogs have been used later on.
Direct measurement of antigen
Antigen
An antigen is a foreign molecule that, when introduced into the body, triggers the production of an antibody by the immune system. The immune system will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. These invaders can be molecules such as...
s, such as hCG, was made possible with the invention of the radioimmunoassay
Radioimmunoassay
Radioimmunoassay is a very sensitive in vitro assay technique used to measure concentrations of antigens by use of antibodies...
in 1959. Radioimmunoassays require sophisticated apparatus and special radiation precautions and are expensive. In the 1970s, the discovery of monoclonal antibodies
Monoclonal antibodies
Monoclonal antibodies are monospecific antibodies that are the same because they are made by identical immune cells that are all clones of a unique parent cell....
led to the development of the relatively simple and cheap immunoassay
Immunoassay
An immunoassay is a biochemical test that measures the presence or concentration of a substance in solutions that frequently contain a complex mixture of substances. Analytes in biological liquids such as serum or urine are frequently assayed using immunoassay methods...
s, such as agglutination-inhibition-based assays and sandwich ELISA, used in modern home pregnancy tests.
Modern tests
The test for pregnancy which can give the quickest result after fertilisation is a rosette inhibition assayEnzyme assay
Enzyme assays are laboratory methods for measuring enzymatic activity. They are vital for the study of enzyme kinetics and enzyme inhibition.-Enzyme units:...
for early pregnancy factor
Early pregnancy factor
Early pregnancy factor or early conception factor is a protein associated with mammalian embryos shortly after fertilization. It may be composed of more than one molecule. It was first described in 1976.-Detection:...
(EPF). EPF can be detected in blood within 48 hours of fertilization
Human fertilization
Human fertilization is the union of a humanoid egg and sperm, usually occurring in the ampulla of the uterine tube. The result of this union is the production of a zygote, or fertilized egg, initiating prenatal development...
. However, testing for EPF is expensive and time-consuming.
Most chemical tests for pregnancy look for the presence of the beta subunit of hCG or human chorionic gonadotropin in the blood or urine. hCG can be detected in urine or blood after implantation, which occurs six to twelve days after fertilization. Quantitative blood (serum beta) tests can detect hCG levels as low as 1 mIU/mL, while urine test strip
Urine test strip
A urine test strip or dipstick is a basic diagnostic instrument used to determine pathological changes in the urine in standard urinalysis. A standard urine test strip may comprise up to 10 different chemical pads or reagents which react when immersed in, and then removed from, a urine sample....
s have published detection thresholds of 20 mIU/mL to 100 mIU/mL, depending on the brand. Qualitative blood tests generally have a threshold of 25 mIU/mL, and so are less sensitive than some available home pregnancy tests. Most home pregnancy tests are based on lateral-flow
Lateral flow test
Lateral flow tests also known as Lateral Flow Immunochromatographic Assays are a simple device intended to detect the presence of a target analyte in sample . Most commonly these tests are used for medical diagnostics either for home testing, point of care testing, or laboratory use...
technology.
With obstetric ultrasonography
Obstetric ultrasonography
Obstetric sonography is the application of medical ultrasonography to obstetrics, in which sonography is used to visualize the embryo or foetus in its mother's uterus...
the gestational sac
Gestational sac
The gestational sac is the only available intrauterine structure that can be used to determine if an intrauterine pregnancy exists, until the embryo is identified....
sometimes can be visualized as early as four and a half weeks of gestation
Gestational age
Gestational age relates to the age of an embryo or fetus . There is some ambiguity in how it is defined:*In embryology, gestational age is the time elapsed since conception. This interval is also termed fertilisation age....
(approximately two and a half weeks after ovulation) and the yolk sac
Yolk sac
The yolk sac is a membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals...
at about five weeks' gestation. The embryo can be observed and measured by about five and a half weeks. The heartbeat may be seen as early as six weeks, and is usually visible by seven weeks' gestation.
Accuracy
Timing of test
False negativeType I and type II errors
In statistical test theory the notion of statistical error is an integral part of hypothesis testing. The test requires an unambiguous statement of a null hypothesis, which usually corresponds to a default "state of nature", for example "this person is healthy", "this accused is not guilty" or...
readings can occur when testing is done too early. Quantitative blood tests and the most sensitive urine tests usually detect hCG shortly after implantation, which can occur anywhere from 6 to 12 days after ovulation
Ovulation
Ovulation is the process in a female's menstrual cycle by which a mature ovarian follicle ruptures and discharges an ovum . Ovulation also occurs in the estrous cycle of other female mammals, which differs in many fundamental ways from the menstrual cycle...
. Less sensitive urine tests and qualitative blood tests may not detect pregnancy until three or four days after implantation. Menstruation
Menstruation
Menstruation is the shedding of the uterine lining . It occurs on a regular basis in sexually reproductive-age females of certain mammal species. This article focuses on human menstruation.-Overview:...
occurs on average 14 days after ovulation, so the likelihood of a false negative is low once a menstrual period is late.
Ovulation may not occur at a predictable time in the menstrual cycle
Menstrual cycle
The menstrual cycle is the scientific term for the physiological changes that can occur in fertile women for the purpose of sexual reproduction. This article focuses on the human menstrual cycle....
, however. A number of factors may cause an unexpectedly early or late ovulation, even for women with a history of regular menstrual cycles. Using ovulation predictor kits (OPKs), or charting the fertility signs
Fertility awareness
Fertility awareness refers to a set of practices used to determine the fertile and infertile phases of a woman's menstrual cycle. Fertility awareness methods may be used to avoid pregnancy, to achieve pregnancy, or as a way to monitor gynecological health....
of cervical
Cervix
The cervix is the lower, narrow portion of the uterus where it joins with the top end of the vagina. It is cylindrical or conical in shape and protrudes through the upper anterior vaginal wall...
mucus or basal body temperature
Basal body temperature
Basal body temperature is the lowest temperature attained by the body during rest . It is generally measured immediately after awakening and before any physical activity has been undertaken, although the temperature measured at that time is somewhat higher than the true basal body temperature...
give a more accurate idea of when to test than day-counting alone.
The accuracy of a pregnancy test is most closely related to the day of ovulation, not of the act of intercourse or insemination that caused the pregnancy. It is normal for sperm to live up to five days in the fallopian tubes, waiting for ovulation to occur. It could take up to twelve further days for implantation to occur, meaning even the most sensitive pregnancy tests may give false negatives up to seventeen days after the act that caused the pregnancy. Because some home pregnancy tests have high hCG detection thresholds (up to 100 mIU/mL), it may take an additional three or four days for hCG to rise to levels detectable by these tests — meaning false negatives may occur up to three weeks after the act of intercourse or insemination that causes pregnancy.
False positives
False positiveType I and type II errors
In statistical test theory the notion of statistical error is an integral part of hypothesis testing. The test requires an unambiguous statement of a null hypothesis, which usually corresponds to a default "state of nature", for example "this person is healthy", "this accused is not guilty" or...
test results may occur for several reasons. These include: errors of test application, use of drugs containing the assay molecule, and non-pregnant production of the assay molecule.
Spurious evaporation lines may appear on many home pregnancy tests if read after the suggested 3–5 minute window or reaction time, independent of an actual pregnancy. False positives may also appear on tests used past their expiration date.
A woman who has been given an hCG injection as part of infertility
Infertility
Infertility primarily refers to the biological inability of a person to contribute to conception. Infertility may also refer to the state of a woman who is unable to carry a pregnancy to full term...
treatment will test positive on pregnancy tests that assay hCG, regardless of her actual pregnancy status. However, some infertility drugs (e.g., clomid) do not contain the hCG hormone.
Some diseases of the liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
, cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
s, and other medical conditions may produce elevated Human chorionic gonadotropin|hCG and thus cause a false positive pregnancy test. These include choriocarcinoma
Choriocarcinoma
Choriocarcinoma is a malignant, trophoblastic and aggressive cancer, usually of the placenta. It is characterized by early hematogenous spread to the lungs...
and other germ cell tumor
Germ cell tumor
A germ cell tumor is a neoplasm derived from germ cells. Germ cell tumors can be cancerous or non-cancerous tumors. Germ cells normally occur inside the gonads...
s, IgA deficiencies
Selective immunoglobulin A deficiency
Selective immunoglobulin A deficiency is a relatively mild genetic immunodeficiency. People with this deficiency lack immunoglobulin A , a type of antibody that protects against infections of the mucous membranes lining the mouth, airways, and digestive tract. It is defined as an undetectable...
, heterophile antibodies
Heterophile
Heterophile is an alternate term to heterosexual and once competed with it for currency, but only did so successfully in Scandinavia. It is a parallel concept to homophile....
, enterocystoplasties, gestational trophoblastic disease
Gestational trophoblastic disease
Gestational trophoblastic disease is a term used for a group of pregnancy-related tumours. These tumours are rare, and they appear when cells in the womb start to grow out of control. The cells that form gestational trophoblastic tumours are called trophoblasts and come from tissue that grows to...
s (GTD), and gestational trophoblastic neoplasms (GTN).
Viability
Pregnancy tests may be used to determine the viability of a pregnancy. Serial quantitative blood tests may be done, usually 2–3 days apart. Below an hCG level of 1,200 mIU/ml the hCG usually doubles every 48–72 hours, though a rise of 50–60% is still considered normal. Between 1,200 and 6,000 mIU/ml serum the hCG usually takes 72–96 hours to double, and above 6,000 mIU/ml, the hCG often takes more than four days to double. Failure to increase normally may indicate an increased risk of miscarriageMiscarriage
Miscarriage or spontaneous abortion is the spontaneous end of a pregnancy at a stage where the embryo or fetus is incapable of surviving independently, generally defined in humans at prior to 20 weeks of gestation...
or a possible ectopic pregnancy
Ectopic pregnancy
An ectopic pregnancy, or eccysis , is a complication of pregnancy in which the embryo implants outside the uterine cavity. With rare exceptions, ectopic pregnancies are not viable. Furthermore, they are dangerous for the parent, since internal haemorrhage is a life threatening complication...
.
Ultrasound
Obstetric ultrasonography
Obstetric sonography is the application of medical ultrasonography to obstetrics, in which sonography is used to visualize the embryo or foetus in its mother's uterus...
is also a common tool for determining viability. A lower than expected heart rate or missed development milestones may indicate a problem with the pregnancy. Diagnosis should not be made from a single ultrasound, however. Inaccurate estimations of fetal age and inaccuracies inherent in ultrasonic examination may cause a scan to be interpreted negatively. If results from the first ultrasound scan indicate a problem, repeating the scan 7–10 days later is reasonable practice.
External links
- A descriptive animation of the principle behind a lateral-flow sandwich-ELISA home pregnancy test.
- A detailed pregnancy due date chart